Relay driver circuit

ABSTRACT

A relay driver circuit is connected between an upper stage relay and a lower stage relay and is configured to be driven in response to driving of the upper relay. The relay driver circuit drives the lower stage relay. The relay driver circuit includes a semiconductor component, a control input line, a protective component, and a buffer circuit. The semiconductor component switches on and off the lower relay. The control input line is electrically connected to a control terminal of the semiconductor component. A power supply voltage is applied to the control input line via the upper stage relay. The protective component is connected in the control input line to protect the semiconductor component. The buffer circuit is connected between the protective component in the control input line and the control terminal of the semiconductor component to compensate for a voltage drop due to the protective component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of PCT/JP2019/009236 filedon Mar. 8, 2019, which claims priority of Japanese Patent ApplicationNo. JP 2018-054225 filed on Mar. 22, 2018, the contents of which areincorporated herein.

TECHNICAL FIELD

The technology described herein relates to a relay driver circuit,specifically, the relay driver circuit capable of maintaining anoperating state of a relay even when a drop occurs in power supplyvoltage.

BACKGROUND

A technology for maintaining an operating state of a relay even when adrop occurs in power supply voltage has been known. An example of such atechnology is disclosed in Japanese Unexamined Patent ApplicationPublication 2014-116197. In Japanese Unexamined Patent ApplicationPublication 2014-116197, a first switch and a second switch are providedin two systems for driving a relay. A resistor is connected between thesecond switch and a coil of the relay. The relay is driven with thesecond switch to which the resistor is connected after a contact of therelay is moved. The relay is driven with the first switch if the dropoccurs in power supply voltage to reduce power consumption and maintainthe operating state of the relay even if the drop occurs in power supplyvoltage. Such a technology is disclosed.

As in Japanese Unexamined Patent Application Publication 2014-116197,semiconductor components are used for driving relays in recent years. Adrop in power supply voltage, which may cause a drop in control inputvoltage applied to the semiconductor components, may have an adverseeffect on control of relay operation.

In FIG. 2, two relays (RL1 and RL2) provided in two stages are connectedto a relay driver circuit 10P. The relay RL2 in the second stage (thelower stage) may be a load connected to the relay RL1 in the first stage(the upper stage). In such a configuration, a diode D1 for protection ofa semiconductor component Q1 may be provided in a control input line LCNconnected to a semiconductor component for driving the lower stage relayRL2. If a load RD1 that is connected in parallel to the lower stagerelay RL2 is an inductive load such as a motor, the diode D1 is providedin the control input line LCN for the protection of the semiconductorcomponent Q1 from a surge caused by the inductive load. The controlinput voltage of the semiconductor component Q1 may drop due to avoltage drop Vf (a forward voltage) of the diode D1. The semiconductorcomponent Q1 may not be able to remain in an on state, that is, theoperating state of the lower stage relay RL2 may not be maintained if adrop in battery voltage Vb (a power supply voltage) is out of tolerance.

A technology described herein is to provide a relay driver circuit thatis capable of maintaining an operating state of a lower stage relay whena power supply voltage is low in a configuration that includes aprotective component provided in a control input line connected to asemiconductor component for driving the lower stage relay that is a loadconnected to an upper stage relay.

SUMMARY

A relay driver circuit described herein is connected between an upperstage relay and a lower stage relay that is configured to be driven inresponse to driving of the upper stage relay. The relay driver circuitis configured to drive the lower stage relay. The relay driver circuitincludes a semiconductor component, a control input line, a protectivecomponent, and a buffer circuit. The semiconductor component switches onand off the lower stage relay. The control input line is electricallyconnected to a control terminal of the semiconductor component. A powersupply voltage is applied to the control input line via the upper stagerelay. The protective component is connected in the control input lineto protect the semiconductor component. The buffer circuit is connectedbetween the protective component in the control input line and thecontrol terminal of the semiconductor component to compensate for avoltage drop due to the protective component.

According to the configuration, the voltage drop due to the protectivecomponent is compensated by the buffer circuit and thus a decrease inpower supply voltage due to the protective component is reduced.Although the configuration for driving the lower stage relay, which is aload connected to the upper stage relay, includes the protectivecomponent provided in the control input line that is connected to thesemiconductor component, an operating state of the lower stage relay ismaintained even if the power supply voltage is low.

In the relay driver circuit, the buffer circuit may include a PNPbipolar transistor and an NPN bipolar transistor. The PNP bipolartransistor may include an emitter that is connected to a power supplyline and a collector that is electrically connected to a controlterminal of the semiconductor component. The NPN bipolar transistor mayinclude a collector that is electrically connected to a base of the PNPbipolar transistor, a base that is electrically connected to an outputof the protective component, and an emitter that is grounded. Theprotective component may include a protective diode that may include ananode that is connected to a contact of the upper stage relay and acathode that is connected to the base of the NPN bipolar transistor.

According to the configuration, a voltage drop (a forward voltage) dueto the diode can be properly compensated by the buffer circuit thatincludes two bipolar transistors. The forward voltage of the diode isabout 0.7 V and a collector-emitter voltage of the PNP bipolartransistor is normally less than 0.7 V. When the power supply voltage isapplied via the PNP bipolar transistor, the power supply voltage with aless voltage drop in comparison to a configuration in which the powersupply voltage is applied via the diode can be applied to the controlterminal of the semiconductor component. Namely, the voltage drop due tothe diode is properly compensated.

In the relay driver circuit, the buffer circuit may include a rectifierdiode that may include an anode that is electrically connected to thebase of the PNP bipolar transistor and a cathode that is electricallyconnected to the collector of the NPN bipolar transistor.

According to the configuration, if the power supply is a battery,protection against inverse connection of the battery is provided.

Advantageous Effect of the Disclosure

According to the relay driver circuit described herein, although theconfiguration for driving the lower stage relay, which is a loadconnected to the upper stage relay, includes the protective componentprovided in the control input line that is connected to the driversemiconductor component, an operating state of the lower stage relay ismaintained even if the power supply voltage is low.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a relay system including a relaydriver circuit according to an embodiment of the present disclosure.

FIG. 2 is a schematic block diagram of a relay system including a knownrelay circuit.

FIG. 3 is a schematic block diagram of a relay system including anotherexample of a relay driver circuit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A relay driver circuit 10, which is an embodiment according to thepresent disclosure, will be described with reference to FIG. 1. Therelay driver circuit 10 in this embodiment is installed in a vehicle anda power supply is a battery Ba that is an onboard battery. The relaydriver circuit 10 and the power supply are not limited to those for avehicle.

Circuit Configuration

As illustrated in FIG. 1, the relay driver circuit 10 is connectedbetween an upper stage relay RL1 and a lower stage relay RL2 that isdriven in response to driving of the upper stage relay RL1. The relaydriver circuit 10 is configured to drive the lower stage relay RL2. Thelower stage relay RL2 is connected to the upper stage relay RL1 and inparallel to an inductive load RD1 including a motor M. The lower stagerelay RL2 and the inductive load RD1 become active when the upper stagerelay RL1 is actuated.

The relay driver circuit 10 includes a semiconductor component Q1, acontrol input line LCN, a protective diode D1, and a buffer circuit 20.

The semiconductor component Q1 may include an N-channel MOSFET asillustrated in FIG. 1. The semiconductor component Q1 is configured toswitch on and off the lower stage relay RL2 in response to on and off ofthe upper stage relay RL1. Specifically, the semiconductor component Q1is configured to start and stop a flow of an exciting current through anexciting coil L2 in the lower stage relay RL2. Bias resistors (R1, R2)are connected to a gate G of the semiconductor component Q1.

The control input line LCN is electrically connected to the gate G ofthe semiconductor component Q1. The gate G is a control terminal. Whenthe upper stage relay RL1 is in the on state, a battery voltage Vb (apower supply voltage) is applied to the control input line LCN via arelay contact 1 of the upper stage relay RL1.

A protective diode D1 (an example of a protective component) is providedin the control input line LCN. The protective diode D1 protects thesemiconductor component Q1 from a surge caused by the inductive loadRD1, such as a motor that is connected to the upper stage relay RL1,transmitted through the control input line LCN. An anode A of theprotective diode D1 is connected to the relay contact 1 of the upperstage relay RL1 via the control input line LCN. A cathode of theprotective diode D1 is connected to a base B of an NPN bipolartransistor TR1, which will be described later, via a bias resistor R3.

The buffer circuit 20 is connected between the protective diode D1 inthe control input line LCN and the gate G of the semiconductor componentQ1 to compensate for the voltage drop Vf (the forward voltage) due tothe protective diode D1.

Specifically, as illustrated in FIG. 1, the buffer circuit 20 includesthe NPN bipolar transistor (hereinafter referred to as the NPNtransistor) TR1, a PNP bipolar transistor (hereinafter referred to asthe PNP transistor) TR2, a rectifier diode D2, and bias resistors (R3,R4, R5, and R6).

An emitter E of the NPN transistor TR1 is connected to a ground so thatthe emitter E is grounded. A collector C of the NPN transistor TR isconnected to a cathode K of the rectifier diode D2. The collector C iselectrically connected to a base B of the PNP transistor TR2 via therectifier diode D2 and the bias resistor R5. The base B of the NPNtransistor TR1 is electrically connected to the cathode K (an output ofthe protective component) of the protective diode D1 via the biasresistor R3.

The emitter E of the PNP transistor TR2 is connected to a power supplyline LV. The collector C of the PNP transistor TR2 is electricallyconnected to the gate G of the semiconductor component Q1 via the biasresistor R1. The base B of the PNP transistor TR2 is connected to theanode A of the rectifier diode D2 via the bias resistor R5.

The anode A of the rectifier diode D2 is electrically connected to thebase B of the PNP transistor TR2 via the bias resistor R5. The cathode Kof the rectifier diode D2 is connected to the collector C of the NPNtransistor. The rectifier diode D2 is a protection against inverseconnection of the battery Ba. If the battery Ba is inversely connected,a current flows from the ground to the power supply line LV via theresistor R4, the base B to the collector C of the NPN transistor TR1,resistor R5, and the resistor R6 in this sequence if the rectifier diodeD2 is not provided. Therefore, an inverse voltage is applied between thebase B and the emitter E of the NPN transistor TR1 and the base B andthe emitter E of the PNP transistor TR2. This may result in damages tothe transistors.

Operation of the Buffer Circuit

Next, operation of the buffer circuit 20 having the above configurationwill be described.

When the upper stage relay RL1 is switched on through actuation of anupper stage relay actuating switch SW1, the battery voltage Vb isapplied to the buffer circuit 20 via the control input line LCN and theprotective diode D1. A base current flows through the NPN transistor TR1and the NPN transistor TR1 turns on. A base current flows through thePNP transistor TR2 and the PNP transistor TR2 turns on.

The battery voltage Vb is applied to the gate G of the semiconductorcomponent Q1 via the PNP transistor TR2. The semiconductor component Q1turns on and thus the lower stage relay RL2 is switched on.

Normally, a collector-emitter voltage drop in the PNP transistor TR2 isequal to or less than 0,1 V, which is sufficiently smaller than thevoltage drop Vf due to the protective diode D1 (normally, 0.7 V). Incomparison to the known example illustrated in FIG. 2, a voltage closerto the battery voltage Vb can be applied to the gate G of thesemiconductor component Q1 (more precisely, bias resistors R1 and R2).In the configuration that includes the protective diode D1 in thecontrol input line LCN, the on state of the semiconductor component Q1is maintained and thus the on state of the lower stage relay RL2 ismaintained even if the battery voltage Vb is further dropped incomparison to the known example.

Effects of the Embodiment

The voltage drop Vf due to the protective diode D1 is compensated by thebuffer circuit 20. Namely, with the buffer circuit 20, a decrease inbattery voltage Vb due to the protective diode D1 can be reduced.Although the configuration for driving the lower stage relay RL2, whichis the load connected to the upper stage relay RL1, includes theprotective diode D1 that is connected in the control input line LCN thatis connected to the semiconductor component Q1 (the driver semiconductorcomponent), the operating state of the lower stage relay RL2 ismaintained even if the battery voltage Vb is lower in comparison to theconfiguration that does not include the buffer circuit 20. This improvesfunctionality and safety of a vehicle system.

The buffer circuit 20 properly compensates for the voltage drop Vf (theforward voltage) due to the protective diode D1 with two bipolartransistors (TR1 and TR2), that is, with a simple configuration.Normally, the forward voltage of a diode is about 0.7 V and thus avoltage drop due to an on-resistance of the PNP bipolar transistor TR2is equal to or less than 0.1 V. When applying the battery voltage Vb tothe gate terminal G (the control terminal) of the semiconductorcomponent Q1 via the PNP bipolar transistor TR2, the battery voltage Vbwith less voltage drop can be applied to the gate terminal G incomparison to a configuration in which the battery voltage Vb is appliedvia the protective diode D1. The voltage drop Vf due to the protectivediode D1 is properly compensated and reduced.

The present disclosure described herein is not limited to the embodimentdescribed above and illustrated in the drawing. For example, thefollowing embodiments will be included in the technical scope of thetechnology described herein.

In the above embodiment, the buffer circuit 20 includes two bipolartransistors (TR1 and TR2). However, the configuration of the buffercircuit 20 is not limited to such a configuration. For example, asillustrated in FIG. 3, the NPN transistor TR1 and the PNP transistor TR2of the buffer circuit 20 may be replaced with an N-channel MOSFET 1 anda P-channel MOSFET 2, respectively.

In the above embodiment, the protective diode D1 is provided as anexample of the protective component. However, the protective componentis not limited to the protective diode D1. For example, if the surgecaused by the inductive load RD1 is significantly small, a resistorhaving a low resistance may be used for the protective component.

In the above embodiment, the rectifier diode D2 is included in thebuffer circuit 20 for the protection against inverse connection of thebattery. However, the rectifier diode D2 may be omitted.

The invention claimed is:
 1. A relay driver circuit connected between anupper stage relay and a lower stage relay configured to be driven inresponse to driving of the upper relay, the relay driver circuitcomprising: a semiconductor component configured to switch on and offthe lower stage relay; a control input line electrically connected to acontrol terminal of the semiconductor component and to which a powersupply voltage is applied via the upper stage relay; a protectivecomponent connected in the control input line to protect thesemiconductor component; and a buffer circuit connected between theprotective component in the control input line and the control terminalof the semiconductor component to compensate for a voltage drop due tothe protective component, wherein the buffer circuit includes a PNPbipolar transistor and an NPN bipolar transistor, the PNP bipolartransistor includes an emitter connected to a power supply line and acollector electrically connected to the control terminal of thesemiconductor component, the NPN bipolar transistor includes a collectorelectrically connected to a base of the PNP bipolar transistor, a baseelectrically connected to an output of the protective component, and anemitter grounded, and the protective component includes a protectivediode including an anode connected to a contact of the upper stage relayand a cathode connected to the base of the NPN bipolar transistor. 2.The relay driver circuit according to claim 2 claim 1, wherein thebuffer circuit further includes a rectifier diode including an anodeelectrically connected to the base of the PNP bipolar transistor and acathode electrically connected to the collector of the NPN bipolartransistor.
 3. A relay driver circuit connected to an upper stage relayconnected to a power supply and to a lower stage relay configured to bedriven in response to driving of the upper stage relay, the relay drivercircuit comprising: a semiconductor component connected to the lowerstage relay and configured to switch on and off the lower stage relay; acontrol input line connected to the upper stage relay and to which apower supply voltage is applied via the upper stage relay; a protectivecomponent connected between the control input line and a controlterminal of the semiconductor component to protect the semiconductorcomponent; and a buffer circuit connected between the protectivecomponent and the control terminal of the semiconductor component tocompensate for a voltage drop due to the protective component, whereinthe buffer circuit includes a PNP bipolar transistor and an NPN bipolartransistor, the PNP bipolar transistor includes an emitter connected toa power supply line and a collector electrically connected to thecontrol terminal of the semiconductor component, the NPN bipolartransistor includes a collector electrically connected to a base of thePNP bipolar transistor, a base electically connected tp an output of theprotective component, and an emitter grounded, and the protectivecomponent includes a protective diode including an anode connected to acontact of the upper stafe relay and a cathode connected to the base ofthe NPN bipolar transistor.
 4. The relay driver circuit according toclaim 3, wherein the buffer circuit includes an input terminal connectedto the protective component and an output terminal connected to thecontrol terminal of the semiconductor component.
 5. The relay drivercircuit according to claim 3, wherein the buffer circuit furtherincludes a rectifier diode including an anode electrically connected tothe base of the FNP bipolar transistor and a cathode electricallyconnected to the collector of the NPN bipolar transistor.